The present invention relates to a battery constructed such that a power generating element is sealed in a film exterior member, such as a lithium ion polymer secondary battery.
Portable electronics devices such as notebook personal computers, integrated camera/VTRs (videotape recorders) or cellular phones have recently come into market one after another, and attempts at miniaturization and weight reduction of the devices are being made. In association with this trend, attention is being given to secondary batteries as a portable power supply, which can be carried around, and intensive studies to attain further higher energy density are being conducted. Amid such studies, a lithium ion secondary battery employing a non-aqueous electrolyte, which has higher energy density than that of a secondary battery with an aqueous electrolyte such as a lead secondary battery and a nickel cadmium secondary battery has been proposed and is being put into use.
Conventionally, with the lithium ion secondary battery, a liquid electrolyte (electrolytic solution) wherein lithium salt, which serves as a conducting ion, is dissolved in a non-aqueous solvent has been used. Therefore, to prevent leakage, hermeticity needs to be securely maintained inside a battery using a metallic case as an exterior member. Use of the metallic case as the exterior member, however, makes it very difficult to fabricate a sheet-type battery with a thin large area, a card-type battery with a small thin area, a flexible battery with an adjustable shape and the like.
To this end, a secondary battery using a gel electrolyte, instead of an electrolytic solution, has been proposed. With this gel electrolyte, a high molecular weight material retains an electrolytic solution containing lithium salt. Such a battery is free from leakage so that laminated film can be used as an exterior member, whereby much smaller, lighter and thinner batteries can be realized and batteries with flexible shapes also become feasible.
However, when laminated film is used as the exterior member, there is a problem such that expansion of the battery caused by gas emission inside the battery is significant when the battery is stored at a high temperature. The greatest cause of the expansion of the battery is vaporization of ingredients of electrolytic solutions. Further, gases emitted in dissolving electrolytic solutions such as hydrogen gas, carbonic acid gas and the like also produce expansion of the battery.
The present invention has been achieved in view of the aforementioned problems. The object of the present invention is to provide a battery which can reduce expansion of the battery due to gas emission occurring when storing at high temperature.